MOS field-effect transistors with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high-k dielectric materials, instead of silicon dioxide, can reduce gate leakage. Because, however, such a dielectric may not be compatible with polysilicon, it may be desirable to use metal gate electrodes in devices that include high-k gate dielectrics. Certain metals with a workfunction below 4.3 eV may be used to make metal gate electrodes for NMOS transistors. Those metals, however, may be thermally unstable at temperatures above 400° C., causing them to react adversely with high-k gate dielectrics.
Accordingly, there is a need for a semiconductor device with a high-k gate dielectric, and an NMOS metal gate electrode with a workfunction below 4.3 eV that is thermally stable at 400° C. The present invention provides such a semiconductor device.
Features shown in these figures are not intended to be drawn to scale.